I consider this one of the less essential books in the series. While the Infernalist is flavorful and does the job, the taint mechanic is not all that inspired, and there is little to set this character apart from a straight-up Mage. That said, the class abilities are useful, and the new feats do add a little oomph to your typical dark sorcerer. I have a hard time picturing this class as having much traction for a PC, and for an NPC, the new material can be summed in the new feats and the new class abilities available as qualities. If you are really interested in a character full of roiling power who consorts with dark powers, this class makes a lot of sense.